This repository describes custom boards designed to replace the P2111 SRAM and MOS 6530-024 RRIOT vintage chips used in the Commodore Chessmate chess computer.
The Commodore Chessmate is a vintage chess computer released in 1978 based on MicroChess by Peter Jennings. It was one of the first commercialy available dedicated chess computers. https://www.chessprogramming.org/Commodore_ChessMate
It was built on a similar architecture as the KIM-1 SBC, based on the following main components:
- MOS 6504 CPU, a simplified 6502 variant https://en.wikipedia.org/wiki/MOS_Technology_6502#Variants
- MOS 6530-024 RRIOT (ROM-RAM-I/O-Timer) https://en.wikipedia.org/wiki/MOS_Technology_RRIOT
- 2 x P2111 SRAM (only 256 Bytes!)
- 6332 ROM (there are variants with 1x4KB ROM and earlier variants with 2x2KB ROMs)
The Chessmate and MicroChess have very interesting stories, even including the participation of Bobby Fischer himself. You can get more information about them from the own author's recollections here (it is a fun read): https://benlo.com/microchess/chessmate.html or from the Commodore International Historical Society here: https://commodore.international/2022/03/26/the-history-of-the-commodore-chessmate/
My unit came with broken P2111 SRAM and 6530 RRIOT. P2221 SRAM is expensive, unreliable and very difficult to source, and the 6530-024 is directly unobtainium, considering that is a variant that is custom-made and factory programmed specifically for the Chessmate, the only possible source of a replacement part is getting one from a donor board.
So I decided to design replacement parts based on easier to get ICs:
- For the RAM I designed a direct adapter for 6264 SRAM, that is not exactly period-correct (was first available around 1982) but it is reasonably close, cheap, reliable and very easy to source.
- Replacing the 6530-024 is more complicated, for the RAM-I/O-Timer part there is a very similar part that is the MOS 6532 RIOT (it is functionally like a 6530 minus the ROM). Other adapters exist from the 6532 to other 6530 variants (like for the 6530-030 on the KIM-1 http://retro.hansotten.nl/6502-sbc/kim-1-manuals-and-software/kim-1-6530-replacement/ http://www.baltissen.org/newhtm/6530repl.htm), but cannot be used directly on the Chessmate because the 6530-024 has a slightly different addressing scheme, so I made a custom adapter board based on those. For replacing the ROM, the ubiquitous and cheap 27C512 was used.
My MOS 6504 CPU and 6332 ROM were fine, but if someone needs parts to replace those, there are adapters already available designed by others that can be found online.
The 6532 RIOT is functionaly similar to the 6530 RRIOT, with a different addressing scheme and minus the ROM (used on the Chessmate to store the openings book). So some logic glue is needed, that is implemented with a couple of 74LS00 NAND gates ICs. The 6532 also lacks an internal pullup on the interrupt line that the 6530 has, so the adapter adds an external pullup resistor.
The 27C512 is a 64KB IC, but only 1KB is needed for the Chessmate opening book. I used a 27C512 because it is the cheaper variant you can get, but 27C256, 27C128, 27C64 or even EEPROM equivalent parts can be used if you have those, and they should work the same.
The adapter replaces BOTH P2111 ICs on the Chessmate with a 6264 SRAM. Only 256 Bytes of the total 8KB available on the 6264 will be used. A 74LS04 inverter is also needed as one of the RAM Chip Select lines has to be inverted.
I assumed that the P2111 ICs are equally spaced between them on all Chessmate boards. Maybe this was a bold assumption, if there are other Chessmate boards out there with different spacings, a further adapter or a custom PCB will be needed so be careful about that.
So before considering using my board directly, please check that the distance between the same pin on each P2111 is approximately 17 millimeters and that they are vertically aligned.
The 6532 is relatively easy to find online, as it was used on many mass-made devices like the Atari 2600, pinball machines, and disk adapters, and it was manufactured for a relative long time (there are third party compatible parts like the Rockwell R6532P). But be aware that there are many fake and defective parts listed out there, so better try to find one from a reliable provider. A defective 6532 could not work at all or work partially, like the machine mostly works but you will get dead lights on the board, or no sound ,if some output lines or their internal registries are defective.
In general the reliabily of the 6264 and 27C512 ICs are vastly superior and there shouldn't be any problem sourcing good parts.
You can get the Chessmate ROM dumps here: http://retro.hansotten.nl/uploads/chessmate/Commodore%20ChessMate%20ROMs.zip
Note that the RRIOT ROM is only 1KB, program it at the last 1KB of the EPROM device. for example, for a 27C512 write the data starting from the 0xFC00 address of your EPROM, and leave the rest empty.
Here is how it looks like programming the EPROM with Xgpro on a TL866II
Here I list the parts needed to build the adapters, apart from the adapter PCB. Note that the specified packings are chosen to match the provided PCB design, but you can use others if you build your own PCB from the Kicad project files.
- 1 x MOS 6532 RIOT or equivalent, in DIP-40 packing
- 1 x 27C512 EPROM, EEPROM or equivalent, in DIP-28 packing
- 1 x DIP-40 (wide) standard IC socket for the adapter (6532)
- 1 x DIP-28 (wide) standard IC socket for the adapter (27C512)
- 2 x 74LS00 (NAND gates). On my PCB design the packing is SOIC-14
- 4 x 100nF decoupling capacitors. On my PCB design the packing is 0805
- 1 x 3Kohm resistor. On my PCB design the packing is 0805
- Standard pin rows, at least for 40 pins
- 1 x DIP-40 (wide) standard IC socket for the Chessmate PCB
- 1 x 6264 RAM or equivalent in DIP-28 packing
- 1 x DIP-28 socket (wide) for the adapter
- 1 x 74LS04 (inverter gates). On my PCB design the packing is SOIC-14
- 2 x 100nF decoupling capacitors. On my PCB design the packing is 0805
- Standard pin rows, at least for 32 pins
- 2 x DIP-16 (narrow) standard IC sockets for the Chessmate PCB
I include already tested Gerber files to order PCBs directly from any PCB manufacturer on the pcb directory.
I provide also SMT assembly files (BOM and CPL) for JLCPCB so that the 74LS00, 74LS04, resistors and capacitors can be ordered to be provided and assembled by them. Of course you can order the bare board and solder them yourself, but in general I prefer to order the boards fully populated of passive components and glue logic ICs.
I also provide the KICAD project files in the Kicad directory, in case that you want to make any modifications to the design and make your own custom PCBs.
If you don't like to order PCBs it is also perfectly possible to build handmade versions using prototyping boards.
To install the replacement boards you need to remove first the defective parts and solder standard IC sockets on its place. Use regular IC sockets like those on my pictures. DO NOT use turned sockets (the kind that have round insertion points for the ICs) because with those you won't have any easy way to insert the adapters easily.
The replacement boards are assembled with pin rows on the back side of the PCB that will interface with the sockets on the board.
The pin rows use to have a longer side and a shorter side, so you have to decide the orientation that you will use to solder them. That is because depending on the exact height of your pin rows and IC sockets, the adapters might be a tight fit vertically when closing the Chessmate case, or the pins may be too short to interface properly on the main board sockets, so you may want to check for those issues before committing to soldering them on the boards.
The pins on the rows can be shortened a little before soldering, or be soldered backwards with the short side facing down. I find that the latter method (soldering with the short side facing down) is the recommended way to go if you are unsure about the adapter fitting or not.
You may also have to bend flat carefully a couple of big capacitors from the main PCB that may block or make difficult the adapters insertion.
I recommend to put also sockets on the front side for the replacement ICs (6532, 27C512, 6264 RAM) to easily swap them if needed.
"Commodore" and "Chessmate" are trademarks of their respective owners. All the trademarks are used nominatively to indicate compatibility, and do not indicate affiliation to or endorsement by the trademark owners.